1. Field of the Invention
The present invention relates to novel azo compounds, inks containing such azo compounds as a recording agent, and an ink-jet recording method and instrument making use of such inks. More specifically, this invention relates specifically to novel azo compounds suitable for use in the preparation of inks which can achieve high image density and improved water resistance when printing on non-coated paper such as woodfree paper, paper for copying, bond paper and paper for reporting, i.e., so-called plain paper, and which can provide images improved in indoor discoloration when recording on recording materials having on their surface an ink-receiving layer composed of a binder and a pigment, i.e., so-called coated paper, and to inks separately containing such azo compounds. This invention further relates to an ink-jet recording method and an instrument making use of such inks.
2. Related Background Art
Water-based inks with a water-soluble dye dissolved in an aqueous medium have heretofore been used as inks for ink-jet recording. Such conventional inks have been required to satisfy the following conditions:
(1) providing recorded images sufficient in density;
(2) having good drying characteristics on a recording material;
(3) providing recorded images on which feathering scarcely occurs;
(4) providing recorded images free from running, even when they come into contact with water, alcohol, etc., or sufficiently readable even when run out (high water resistance);
(5) providing recorded images excellent in light fastness;
(6) causing no clogging on the tip of a pen or nozzle;
(7) having no defects such as blurring of recorded images upon continuous recording or resumption of recording after being left for a long period of time (good jetting stability);
(8) being able to stored stably;
(9) raising no problem even when coming into contact with parts of a recording means during use; and
(10) offering no problem with respect to the safety of users. In addition, an ink-jet recording system making use of thermal energy requires:
(11) having excellent heat resistance and not adversely affecting a means for generating thermal energy.
These properties or characteristics may of course vary according to the composition of inks, and also are greatly controlled by the nature of a dye contained as a recording agent. It is easy to satisfy all the requirements for performances above by using conventional dyes.
As specific examples of such dyes, C.I. Food Black 2 has been principally used for the formation of both mono-color and full-color images, for example, in a field of ink-jet recording method (see Japanese Patent Application Laid-Open Nos. 59-93766 and 59-93768).
Inks making use of C.I. Food Black 2 are excellent in that sufficiently high-density images are provided, but a problem still remains with respect of light fastness and water resistance. Namely, the color of images printed with such an ink is changed from black to brown when exposed to indoor light for a long period of time or posted near a copying machine, resulting in remarkably deteriorated image quality (this very often occurs where the recording material is a coated paper sheet). A further problem is that the images become unreadable when water is poured out on to the images.
Japanese Patent Application Lid-Open Nos. 55-144067 and 61-285276 disclose inks, the storage stability and jetting responsibility of which have been improved by specifying the structure of a dye used. However, the water resistance of recorded images formed with such inks is still insufficient. In addition, the above-described problem of discoloration is not yet solved with respect to the former inks.
Further, Japanese Patent Application Laid-Open No. 1-135880 discloses inks, the jetting stability, water resistance of recorded images, etc. of which have been improved by introducing at least one specific structural unit in the structure of a dye used. In Japanese Patent Application Laid-Open No. 1-193375, black inks are disclosed having high affinity for recording materials, excellent fixing property and water resistance, and to capability of imparting good quality to images printed on plain paper. Japanese Patent Publication No. 62-10274 discloses inks taking account of jetting stability, light fastness of recorded images, etc.
In Japanese Patent Application Laid-Open Nos. 3-781 and 3-782, are disclosed inks that as hard to clog and scarce in discoloration. Japanese Patent Application Laid-Open Nos. 3-294366 and 3-294367 disclose inks that scarcely undergo discoloration.
Inks first require performances suited to the employed recording systems. After satisfying such performances, the inks are secondly required to exhibit excellent performance as a print, i.e., sufficient performance with respect to quality and fastness properties. However, it is considerably more difficult to satisfy all the above requirements than is apparent from the prior art.
Discoloration has heretofore not been recognized as a serious problem, but has come to attract more attention as a result of the fact that many other problems have been solved through advancement of technique. In particular, the problem is important for black inks which are used to a greater extent. In the case of full-color images, the discoloration of black ink rapidly lowers the quality of the images.
This discoloration proceeds even in a room not exposed to direct sunlight, and varies according to the kind of recording materials on which images are formed. Namely, the discoloration occurs remarkably in paper sheets containing a pigment such as silica. It has been impossible to solve this problem in C.I. Food Black 2, which has been widely used to date.
It has been attempted to solve the deficiency of C.I. Food Black 2 by choosing a dye having good light fastness. Satisfactory results have been accomplished as to plain paper. However, in the case of so-called coated papers obtained by forming an ink-receiving layer containing a pigment and a binder on a base material such as a paper web so as to enhance the image qualities of dye, such as color-producing property, brightness and resolution, significant discoloration has occurred even when inks, which have scarcely involved the problem with respect to the plain paper, have been used. Therefore, it has been impossible to solve such a problem by simply choosing a dye having good light fastness.
Besides, Japanese Patent Application Laid-Open No. 55-152747 discloses compounds similar to the compounds according to this invention, which serve to dye cellulosic fibrous materials, paper or leather black.